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Reverse Osmosis (;) a When pure waler and a sal[ solutlon are w::. on opposite sides of a semipermeable memo W beane. the pure water diffuses through the membrane and dilutes the salt solution. This phenomenon is known us osmosis. Becauseoflhe difference in salt concemra. lion. pure water flows through the memo brane as [hough a pressure were being applied to it. The effective driving force causing the flow is called osmotic pressure. The magnitude of the osmotic pressure depends on the concentration of the salt solution and the temperature of the .......ater. By exerting pressure on the salt solution. the osmosis process can be reversed. When the pressure on the salt solution is greater than the osmotic 'pressure. fresh water diffuses through the membrane in the opposite direction to normal osmotic flow. The princJple of reverse osmosis is ilIus. trated in figure 15. It can readily be seen how this prinCiple can be applied In the conversion of saline water. A diagram of the reverse-osmosis process Is shown In figure 16. The sait water is first pumped through a filter where suspended solids that would damage the membranes are removed. The salt water Is then raised to operating pressure by a second pump and then introduced into the desalination unit. A portion of the water permeates the membranes and is collected as product water at the bottom of the unit. The brine Is discharged althe tOp of the unit. When de- sired. some of the brine may be mixed with incoming saline water and recirculated. process designs Operating plants carry ou t the re- verse-osmosis prinCiple in several different process designs such as the plate-and- frame. tubular. spiral. wound membrane module. and hollow-fiber designs. All are based on the common principle that the membrane. a lIexible plastic film usually no more than 4 to 6 mils in thickness. must have a finn suPPOrt to withstand the very high pressure drop across ii. The prefened material used for membranes in these process cfesigns is cellulose acetate which has been speciaUy processed to make it reject salt and at the same time pass water at a reasonable rate. ..01<...., O\''OIU O~MOIIC IOI.IlI"I,u.. rl..U\I O~o.IOsn --. ::-..:-:.. - ._.-:..F':..~ -- "'~'.. - ,~ - . - ::-.:..:-: _ ----:- -r-~ '.. ! 111\- 1 ~.lll" ,.UI' \ 1\.111 ",""."I-.Ilt MlMI.UIl Fig. 15. Prznciple of relJers~ osmosis. S........II1I. '~"..' -U"o'. "Ill- ,;.:....: ~:;'.:~.~'"'~"..~" r-:~.I:.:....= ;.;;:. C"."II( ........... '1I11(~1 - 'j 1--' . .-. .-=:"".': =:- -.--. -.-.- -.-' ---- - -. _._.- -,- ._-. - -. .~----: .:: '.,. ".'1. Fig. 16. Sc1I~",nri(" dingrnnr ofth~ rC''IJ~rU-OSnlosis proC"rss.